Mechanistic studies are being carried out on three enzymes, bovine cardiac muscle protein kinase, rabbit skeletal muscle phosphofructokinase, and rabbit kidney (Na ion , k ion)-ATPase. These enzymes catalyze phosphate transfer from ATP to different acceptors. We are concentrating our efforts on determining the constitution of the active sites and regulatory sites and on the nature of the reaction intermediates formed. Since the protein kinase and phosphofructokinase are activated by 3' , 5' -cyclic AMP, our investigation of the regulatory properties of the enzymes should provide useful new information on the molecular interaction of 3' ,5'-cyclic AMP with isolated enzymes. (Na ion,K ion) AtPase is commonly believed to be the ion transport system, or at least part of it, which underlies excitability in nerve and muscle. Our studies of the rabbit enzyme should contribute to the development of a model Na ion ,K ion transport, perhaps involving sequential conformational changes in the enzyme which effect the ion translocations.